I have a couple of questions on how BMW did serial numbers on their pre war engines.

I'm assembling a twin cam engine (r5/r51). The left head has what looks like an engine serial number close to the valve cover (# 501136). The right head doesn't have any equivalent number. Both heads have a number stamped near the exhaust of 33656.

I also have another engine case with a serial number 506168 on the left above the push rods, but it doesn't look like it has the BMW roundel. The number on the case is very faint but readable. No hint of the roundels.

The head marking you describe is consistent with what the factory did - the barrels are actually numbered as well on the base. The Engine stamping during this era was very faint and I have seen many with invisible roundels - The un-marked head is likely from BMW stock and didn't leave the factory on a motorcycle.

I am building a 1938 R51 motor now - let me know if you need anything.